Rework boundary parsing for the multipart body (fix #137)

[artemklevtsov]

Summary:

• Rename C++ exported functions (add cpp_ prefix) to avoid collisions with private methods
• Fix boundary parsing and matching
• Add echo demo app (useful to debug) Should fix #137

[lintr-bot]

inst/tinytest/test-parse-multipart.R:80:1: style: Trailing blank lines are superfluous.

​
^

[lintr-bot]

inst/tinytest/test-parse-multipart.R:80:1: style: Trailing blank lines are superfluous.

​
^

[codecov[bot]]

Codecov Report

Merging #139 into dev will increase coverage by 2.35%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #139      +/-   ##
==========================================
+ Coverage   92.54%   94.90%   +2.35%     
==========================================
  Files          27       27              
  Lines        1315     1275      -40     
==========================================
- Hits         1217     1210       -7     
+ Misses         98       65      -33     

Impacted Files	Coverage Δ
R/BackendRserve.R	94.62% <100.00%> (+23.30%)	:arrow_up:
src/format_cookies.cpp	100.00% <100.00%> (ø)
src/format_headers.cpp	100.00% <100.00%> (ø)
src/parse_cookies.cpp	100.00% <100.00%> (ø)
src/parse_headers.cpp	100.00% <100.00%> (ø)
src/parse_multipart.cpp	97.61% <100.00%> (-1.17%)	:arrow_down:
src/url_decode.cpp	95.65% <100.00%> (ø)
src/url_encode.cpp	100.00% <100.00%> (ø)
src/nonstd/string_view.hpp	96.00% <0.00%> (-0.78%)	:arrow_down:
R/Request.R	89.56% <0.00%> (+1.73%)	:arrow_up:
... and 1 more

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[dselivanov]

Since we have issue with upper case boundary it makes sense to add test where boundary has symbols in upper case. For example gc0p4Jq0M2Yt08jU534c0p as used in rfc1341.

[lintr-bot]

inst/tinytest/setup.R:24:7: style: Use =, not <-, for assignment.

​  sym <- c(0:9, letters, LETTERS)
      ^